Bridge 01: MPU MCU Bridge Message

This exercise is a continuation of the Bridge Blink exercise. In this exercise, we will send a message from the Python side to the Arduino side via the Bridge.

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How It Works

1. Python constructs a message — a plain string ("Hello world") is defined on the MPU (Linux) side.
2. Python prints it locally — print() outputs it to the App Lab console so you can confirm what's being sent.
3. Python calls the Arduino — Bridge.call("receive_message", message) sends the string across the Bridge to the Arduino, triggering the receive_message function registered on the MCU side.
4. Arduino receives it — the receive_message(String msg) function fires with the string as its argument.
5. Arduino prints it to the shared Monitor — Monitor.println(msg) outputs it to the shared console, so you can see it arriving on the Arduino side.
6. Arduino does nothing else — loop() is empty. The MCU only acts when Python tells it to.

The key idea: data flows one way here — from MPU to MCU. Python is the sender, Arduino is the receiver. In later exercises we'll flip this and read data back from the Arduino.

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The Code

main.py — Python side

from arduino.app_utils import *
import time

def loop():
    message = "Hello world"

    # Python (MPU) prints this locally to the console
    print(f"MPU sending \"{message}\"")

    # Send the string over the bridge to the Arduino
    Bridge.call("receive_message", message)

    time.sleep(2)

App.run(user_loop=loop)

sketch.ino — Arduino side

#include <Arduino_RouterBridge.h>

// This function receives the message from Python
void receive_message(String msg) {
  // MCU prints this back to the shared console
  Monitor.print("MCU says: ");
  Monitor.println(msg);
}

void setup() {
  Bridge.begin();
  Monitor.begin(); // Turns on the shared console connection

  Bridge.provide_safe("receive_message", receive_message);
}

void loop() {}

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note

Event-based vs Continuous Loop In a typical Arduino sketch, logic lives inside loop() and runs forever. Here, loop() is empty. The Arduino only reacts when Python sends a command via the Bridge. This makes the system more predictable and easier to reason about — Python is the "brain", Arduino is the "muscle".

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